An electric double layer capacitor that is known as one of energy storage devices is generally constituted of a pair of polarizable electrodes each containing a porous material, a separator, an electrolyte solution and the like. This electric double layer capacitor is a device which makes use, as a charge and discharge mechanism, of an electric energy ascribed to the electric double layer established through ionic movement at the interface between the electrodes. Because no electrochemical reaction of an electrode active material is involved, the capacitor does not have such a life as of secondary cells, along with characteristic features including excellent instantaneous charge and discharge characteristics, stable charge and discharge characteristics kept over a wide temperature range, and a reduced lowering of performance in repeated use.
It has been hitherto accepted that the electrostatic capacitance of an electric double layer capacitor has a proportional relation with surface areas of polarizable electrodes. Accordingly, porous materials having a large specific surface area have been studied for use as a polarizable electrode in order to increase the capacitance.
More particularly, the polarizable electrode has been usually made by mixing a porous material such as a carbonaceous material or the like, acetylene black used as a conductive auxiliary agent, and a fluorine polymer or rubbery polymer to obtain an electrode composition, and applying the electrode composition onto a current collector. For instance, attempts have been made to enhance the electrostatic capacitance by using, as a carbonaceous material, active carbon (or a porous carbonaceous material) that exhibits high electric conductivity, is relatively stable in electrochemical aspect and has a large specific surface area.
More particularly, a carbonaceous material, such as coal, coal coke, coconut shell, wood flour, resins and the like, is subjected activation (porous treatment) with an oxidative gas such as steam, air, oxygen, CO2 or the like or by means of a chemical such as zinc chloride, potassium hydroxide or the like, thereby forming fine pores therein. The resulting active carbon with a large surface area has been used.
In recent years, as developments in electronics devices, electric cars and the like are being in progress, the fundamental design of energy storage devices including an electric double layer capacitor is also being changed.
For instance, an electric double layer capacitor needs to have an energy highly densified and be small in size and light in weight. Hence, it becomes necessary to design the capacitor so that not only an electrostatic capacitance per unit weight (F/g) of porous material, but also an electrostatic capacitance per unit volume (F/cm3) is improved (see Patent Document 2: JP-A 2000-68164; Patent Document 3: JP-A 2000-100668; and Patent Document 5: JP-A 11-214270).
The electrostatic capacitance per unit weight of a porous material (polarizable electrode) can be increased by using such a porous material with a large surface area as set out hereinabove.
However, as the specific surface area increases, the density (fill rate) of a porous material lowers. In this sense, the electrostatic capacitance per unit volume is not always in proportional relation with an increase in specific surface area. In fact, it is known that when the specific surface area increases to or over a certain extent, the electrostatic capacitance per unit volume tends to lower.
Thus, when using only the procedure of trying to increase the specific surface area of a porous material, limitation is placed on the increase of the electrostatic capacitance of an electric double layer capacitor, thus making it difficult to attain the high densification of an energy to a level required in recent years (see Patent Document 1: JP-A 11-317333 and Patent Document 4: JP-A 11-297577).
On the other hand, developments have been made on energy storage devices such as polymer cells or capacitors using conductive polymers as an electrode active material.
Where positive and negative electrodes are, respectively, made of a conductive polymer of a similar type, it is limited to broaden a reaction potential depending on the oxidation-reduction potential of the positive and negative electrodes. Thus, it is generally difficult to make a polymer cell or capacitor which works at high voltage.
Polythiophene is a substance whose HOMO (highest occupied molecular orbital) and LUMO (lowest occupied molecular orbital) are, respectively, observed at oxidation side and reduction side positions of about 0.7 V and about 2.3 V when measuring by use of a silver/silver oxide electrode as a reference electrode. From this, it can be expected that this compound exhibits wide potential activity under conditions where a conductive polymer of a similar type is used for the positive and negative electrodes, respectively. Thus, studies have been made on an electrode using polythiophene to provide a wide voltage range (see Non-Patent Document 1: Journal Power Source).
Further, a polymer cell or capacitor has been already developed wherein different types of conductive polymers are used as positive and negative electrodes, respectively, in such a way that a conductive polymer more susceptible to oxidation is used as a positive electrode and a conductive polymer more susceptible to reduction is used as a negative electrode. The cell or capacitor is usable over a wide voltage range with the capacitance being high (see Patent Document 6: JP-A 2002-134162). In this cell or capacitor, poly-5-cyanoindole is used as a positive electrode active material, and polyphenylquinoxaline is used as a negative electrode active material.
However, since this energy storage device needs to use different types of molecules in the positive and negative electrodes as set out hereinabove, this is defective from the standpoint of productivity. Thus, there is a demand for development of a conductive polymer compound that can be used as both positive and negative electrodes and can serve as an electrode active material showing wide potential activity.                Patent Document 1: JP-A 11-317333        Patent Document 2: JP-A 2000-68164        Patent Document 3: JP-A 2000-100668        Patent Document 4: JP-A 11-297577        Patent Document 5: JP-A 11-214270        Patent Document 6: JP-A 2002-134162        Non-Patent Document 1: Journal Power Source, Vol. 47, page 89, 1994        